PURPOSE: To offer the means for preventing seizure of a roll at the time of forming cylindrically a metallic hoop by the roll.

CONSTITUTION: The lower roll of forming rolls is divided into two. As for the lower rolls 2 divided into two, the direction being orthogonal to the advance direction of a metallic hoop and the rotation axis form an inclination angle β, and this inclination angle β can be adjusted. Also, as for this lower roll, the width direction of the metallic hoop and the rotation axis form a cross angle α in the plane being orthogonal to the advance surface of the metallic hoop. At the time of forming the metallic hoop, a relation of the inclination angle β, a feed speed V of the metallic hoop, wall thickness T of the metallic hoop, and yield strength σy of the metallic hoop is set to sinβ.V.σy.T2≤25000(m/min.kg/mm2.mm2). In such a way, a welded tube whose surface quality is satisfactory is obtained.